The invention concerns an apparatus for the metered delivery of fluids, as set forth in the classifying portion of claim 1.
Apparatuses for the metered delivery of media which are capable or being delivered, such as a fluid, are also referred to for the sake of brevity as `metering pumps`. In contrast to continuous-flow pumps which for example physically displace quantities of fluid in a continuous flow, the function of metering pumps lies in making delivery media available in given amounts, that is to say metered quantities, at fixed intervals of time. A distinction is made in relation to pumps of that kind between valved metering pumps and valve-less metering pumps, with the valve-less metering pumps being in the forefront for dealing with small amounts, that is to say small metered quantities. The present invention is concerned with metering pumps of the latter kind (referred to hereinafter as pumps for the sake of brevity).
In such pumps, the actual delivery unit, usually a piston-cylinder unit, has to perform two functions, firstly the function of providing for pure delivery and secondly measuring out the discharged metered amounts. The delivery action is effected by axial displacement of the piston (stroke movement) and the measuring-out function is achieved, utilising the stroke movement, by virtue of rotation (stroke movement+rotation) of tile piston, with which inlet and outlet openings or ports in the cylinder are respectively opened and closed. The rotary movement of the piston is effected by means of a rotary drive and the stroke movement is produced by way of a sensing or follower pin which follows a control cam during the rotary movement.
A known pump includes a piston-cylinder unit with inlet and outlet extending transversely with respect to the axial direction, and a piston which is rotatable and axially slidable in the cylinder. The end of the piston which projects out of the cylinder is accommodated in a coupling which in turn co-operates with a rotary drive. On its end face which is towards the cylinder, the coupling carries the pin which follows the control cam. In that way the coupling is displaced axially by the same distance as the piston. The control ram (referred to hereinafter as the slide face) is arranged at the annular end face of the cylinder which is disposed in opposite relationship to the coupling and over which the pin then passes (the end face is normally ground and lapped at an angle of 1.5 degrees).
In the known pump, it is already possible to find severe amounts of wear at the sealing surface of the piston after a relatively short period of time. Such wear phenomena are due to a lateral piston pressure which in turn results from the rigid connection of the piston to the pin (referred to hereinafter as the slide shoe). The arrangement of the slide face on the cylinder by grinding and lapping same is complicated and expensive from the production procedure point of view, and in the event of the slide face exhibiting wear phenomena, the cylinder has to be replaced as an entire and expensive unit. Forming the slide face on the end face of the cylinder incurs the further disadvantage that wetting of the slide face with fluid to be delivered, possibly issuing from the cylinder bore due to the sealing conditions involved, is not impossible. Such wetting results in contamination and soiling of the slide face and, in the case of aggressive fluids being delivered, it results in damage to or destruction or the slide shoe. The angle at which the slide face is disposed for example relative to the outer peripheral wall of the cylinder (skew or bevel angle) determines the stroke length or travel or the piston in the cylinder and, over travel distances of different lengths, the delivery range of the known pump. The known pump does not lend itself to conversion to smaller or larger amounts of delivery media, as for that purpose at least the cylinder but in most cases the entire piston-cylinder unit would have to be replaced, with a modified slide face angle.
It is known that, in the case of a pump of the known kind, the viscosity of the delivery medium and the speed or rotation of the piston determine the delivery amount per revolution (that is to say the metered amount delivered). With the speeds of rotation remaining the same and with the viscosity being unchanged, the delivery amount per revolution is constant. Constancy in terms of delivery amount is an important operational criterion for pumps of the kind referred to herein.
In consideration of the electrical power supply to the rotary drive, speeds of rotation of the piston can fluctuate, while the viscosity of the medium being conveyed can also change, for example under the influence of temperature. The known pump does not provide any means for being able to make adjustments during operation of the pump to keep the amounts delivered per revolution constant.
Taking the known pump as its basic starting point, the object of the present invention is to provide a pump which constitutes a development in relation to the known pump, and that object is attained by a pump having the features of the claims individually or in combination.